Multibit Cell of Magnetic Random Access Memory With Perpendicular Magnetization

ABSTRACT

A multi-bit cell of magnetic random access memory comprises a magnetic tunnel junction element including a first and second free layer comprising a changeable magnetization oriented substantially perpendicular to a layer plane in its equilibrium state and a switching current, a first and second tunnel barrier layer, and a pinned layer comprising a fixed magnetization oriented substantially perpendicular to a layer plane, the pinned layer is disposed between the first and second free layers and is separated from the free layers by one of the tunnel barrier layers, a selection transistor electrically connected to a word line, and a bit line intersecting the word line. The magnetic tunnel junction element is disposed between the bit line and the selection transistor and is electrically connected to the bit line and the selection transistor, wherein the first and second free layers have substantially different switching currents.

CROSS-REFERENCE TO RELATED APPLICATION

This invention claims benefit of U.S. Provisional Patent Application No.61/231,574 entitled “Multi-bit Magnetic Random Access Memory Cell” filedAug. 5, 2009, which is hereby incorporated by reference in its entirety.

FEDERALLY SPONSORED RESEARCH

Not Applicable

SEQUENCE LISTING OR PROGRAM

Not Applicable

FIELD OF THE INVENTION

The present invention relates to a magnetic random access memory (MRAM)and, more specifically, to multi-bit memory cell of MRAM with aperpendicular magnetization.

BACKGROUND OF THE INVENTION

Magnetic random access memory (MRAM) is a new memory technology thatwill likely provide a superior performance over existing semiconductormemories including flash memory and may even replace hard disk drives incertain applications requiring a compact non-volatile memory device. InMRAM bit of data is represented by a magnetic configuration of a smallvolume of ferromagnetic material. Magnetic state of the ferromagneticmaterial can be measured during a read-back operation. The MRAMtypically includes a two-dimensional array of memory cells wherein eachcell comprises one magnetic tunnel junction (MTJ) element that can storeat least one bit of data, one selection transistor (T) and intersectingconductor lines (so-called 1T-1MTJ design).

Conventional MTJ element represents a patterned thin film multilayerthat includes at least a pinned magnetic layer and a free magnetic layerseparated from each other by a thin tunnel barrier layer. The free layerhas two stable orientations of magnetization that are parallel oranti-parallel to the fixed orientation of magnetization in the pinnedlayer that correspond to two logic states “0” or “1”. Resistance of theMTJ element depends on the mutual orientation of the magnetizations inthe free and pinned layers and can be effectively measured. A resistancedifference between the parallel and anti-parallel states can exceed 600%at room temperature.

FIG. 1 shows a schematic view of memory cell 10 for storing four logicstates according to a prior art disclosed in U.S. Pat. No. 5,930,164(Zhu). The cell 10 includes two MTJ elements 11 and 12 connected inseries and magnetically separated from each other by a conductive layer13 made of a non-magnetic material. First MTJ element 11 comprises afirst pinned layer 111 and a first free layer 112 made of CoFe andNiFeCo, respectively. Both the layers 111 and 112 are about 50 Å thick.A tunnel barrier layer 113 separates the layers 111 and 112 from eachother. The layer 113 is made of Al₂O₃ and has a thickness of 22-30 Å.Second MTJ element 12 has a second pinned layer 121 and a second freelayer 122 separated from each other by a second barrier layer 123. Thesecond pinned layer 121 and the second free layer 122 have 50 Å and 30 Åin thickness, respectively. The second free layer 122 is thinner thanthe first free layer 112. This difference provides the free layers 112and 122 of the MTJ elements 11 and 12 with different hysteresis (orswitching) characteristics. The second tunnel barrier layer 123 is madethinner than the first tunnel barrier layer 113. That results indifferent resistance values of MTJ elements 11 and 12. Thickness of thelayer 123 is in a range of 15-22 Å. The pinned layers 111 and 121 aremagnetically pinned by anti-ferromagnetic layers (not shown), which areplaced adjacent their external surfaces.

A current source 14 is coupled to the MRAM cell 10 to provide a sensecurrent 15 through the MTJ elements 11 and 12 to a common groundterminal 16. A resistance over cell 10 varies according to the magneticstates of the free layers 112 and 122; thereby a voltage output V_(OUT)over the MRAM cell 10 indicates different values. The output signalV_(OUT) is compared to threshold voltages, which are predetermined fromhysteresis characteristics of the cell 10 for identification of recordeddata. One of several disadvantages of the cell 10 is a largelength-to-width aspect ratio of the MTJ elements 11 and 12 thatsubstantially reduces a storage density of MRAM chip.

FIG. 2 shows a schematic view of a magnetoresistive element 20comprising two MTJ elements 11 and 12 according to another prior artdisclosed in U.S. Pat. No. 6,590,806 (Bhattacharyya). The element 20distinguishes from the cell 10 shown in FIG. 1 by using a common pinnedlayer 22 for two MTJ elements 11 and 12. The pinned layer 22 has astructure of a synthetic antiferromagnet (SAF). The SAF pinned layer 22is composed of two magnetic layers 111 and 121 antiferromagneticallycoupled to each other through 0.5-1.0 nm thick layer 221 of Ruthenium(Ru) or Copper (Cu). The SAF structure of the pinned layer 22 allows areduction of length-to-width aspect ratio. However this reduction is notsufficient for high density MRAM.

Both MRAM elements according to prior arts shown in FIG. 1 and FIG. 2employ field induced switching mechanism of the free layers 112 and 122that is based on use of two orthogonal magnetic fields. The fieldinduced switching mechanism suffers from high write current, a large andcomplicated cell design and causes a serious half-selected cells problemin MRAM array and in the memory cells with two free layers, especially.Besides, the memory elements 10 shown on the FIGS. 1 and 20 shown on theFIG. 2 employ magnetic materials with in-plane magnetization that limittheir thermal stability a scalability at technology node below 90 nm.

What is needed is a simple design of multi-bit memory cell having a highthermal stability, small cell size, excellent scalability and lowswitching current; the memory cell that does not suffer from ahalf-selection problem.

SUMMARY OF THE INVENTION

The present invention provides a multi-bit cell of magnetic randomaccess memory with perpendicular magnetization.

A magnetic memory cell according to an aspect of the present inventioncomprises a magnetic tunnel junction element including a first andsecond free layer comprising a changeable magnetization orientedsubstantially perpendicular to a layer plane in its equilibrium stateand a switching current, a first and second tunnel barrier layer, and apinned layer comprising a fixed magnetization oriented substantiallyperpendicular to a layer plane, the pinned layer is disposed between thefirst and second free layers and is separated from the free layers byone of the tunnel barrier layers; a selection transistor electricallyconnected to a word line, and a bit line intersecting the word line; themagnetic tunnel junction element is disposed between the bit line andthe selection transistor and is electrically connected to the bit lineand the selection transistor, wherein the first and second free layershave substantially different switching currents.

A method of writing to a magnetic random access memory according toanother aspect of the present invention comprises: providing a magnetictunnel junction element including a first and second free layercomprising a changeable magnetization oriented perpendicular to a layerplane in its equilibrium state and a switching current, a first andsecond tunnel barrier layer, and a pinned layer comprising a fixedmagnetization oriented substantially perpendicular to a layer plane; thepinned layer is disposed between the first and second free layers and isseparated from the free layers by one of the tunnel barrier layers;driving a bias current pulse through a bit line in a proximity to butnot through the magnetic tunnel junction element for producing a biasmagnetic field along a hard magnetic axis of the pinned, first free andsecond free layers, and driving a switching current pulse through themagnetic tunnel junction element along an easy axis of the pinned, firstfree and second free layers for producing a spin momentum transfer,wherein the switching current pulse substantially superimposes the biascurrent pulse, and the first and second free layers have substantiallydifferent switching currents.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic cross-section view of multi-bit MRAM cell within-plane magnetization in free and pinned layers according to a priorart.

FIG. 2 shows a schematic cross-sectional view of multi-bitmagnetoresistive element with one pinned layer having a structure of asynthetic antiferromagnet according to another prior art.

FIG. 3 shows a schematic cross-sectional view of a multi-bit memory cellwith a perpendicular magnetization according to an embodiment of thepresent invention.

FIG. 4 illustrates a table of resistance values corresponding torelative orientation of magnetization in the free layers of the memorycell shown in FIG. 3.

FIG. 5 is a graph illustrating a simulated dependence of normalizedmagnitude of a spin-polarized switching current on tilting angle ofmagnetization in the free layer relatively to an axis perpendicular to alayer plane.

FIG. 6 shows a schematic cross-section view of multi-bit MTJ elementwith two pinned layers according to another embodiment of the presentinvention.

FIGS. 7A-7B show schematic cross-sectional views of multi-bit MTJelement with enhanced spin polarization of pinned layer according to yetother embodiments of the present invention.

FIG. 8 shows a schematic cross-sectional view of multi-bit perpendicularMTJ element with enhanced spin polarization according to still anotherembodiment of the present invention.

FIG. 9 is a schematic view of MRAM module including multi-bit memorycells shown in FIG. 3.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the following detailed description of the preferred embodiments,reference is made to the accompanying drawings that form a part hereof,and in which are shown be way of illustration specific embodiments inwhich the invention may be practiced. It is understood that otherembodiments may be utilized and structural changes may be made withoutdeparting from the scope of the present invention.

The leading digits of reference numbers appearing in the Figuresgenerally corresponds to the Figure number in which that component isfirst introduced, such that the same reference number is used throughoutto refer to an identical component which appears in multiple Figures.

FIG. 3 shows schematic cross-sectional view of memory cell 30 accordingto an embodiment of the present invention. The cell 30 includes amagnetoresistive (MR) element 31, a bit line 33, a word line 34 and aselection transistor 35. The MR element 31 is placed between the bitline 33 and the selection transistor 35 and is electrically connectedwith them in series by means of conducting seed 36 and cap 37 layers.The word line 34 is connected to a gate of the transistor 35 andintersects the bit line 33. The MR element 31 comprises two magnetictunnel junction (MTJ) elements 11 and 12 connected in series. The MTJelement 11 includes a free layer 112 with changeable magnetization M₁₁₂(shown by arrow) that is oriented substantially perpendicular to a layerplane in its equilibrium state, a pinned layer 32, and a tunnel barrierlayer 113 disposed between the layers 112 and 32. The pinned layer 32has a fixed magnetization M₃₂ that is oriented substantiallyperpendicular to a layer plane. The MTJ element 12 comprises a freelayer 122 with a changeable magnetization M₁₂₂ (shown by arrow) that isoriented substantially perpendicular to a layer plane in its equilibriumstate, the pinned layer 32 and a tunnel barrier layer 123 placed betweenthe layers 123 and 32.

In the equilibrium state the magnetizations M₁₁₂, M₃₂ and M₁₂₂ arecollinear (parallel or antiparallel to each other). To write a data tothe free layers 112 or 122 a spin-polarized current I_(S) is applied tothe MR element 31 in direction perpendicular to a layer plane. Thespin-polarized current I_(S) produces a spin momentum transfer in thefree layers 112 and 122 and might cause a magnetization reversal in thefree layers from up to down orientation or vice-versa. The orientationof magnetization in the free layers 112 and 122 is controlled by thedirection of the spin-polarized current I_(S). To reverse themagnetization in the free layer the magnitude of the spin-polarizedcurrent should exceed a critical current that depends on volume andmagnetic properties of the free layer, and other parameters. Thecritical current of the spin-transfer reversal in free layer of MTJelement with perpendicular magnetization at zero temperature is given by

$\begin{matrix}{{I_{c\; 0} = {{- \left( \frac{2e}{\hslash} \right)}\frac{\alpha \; M_{S}V}{{g(\theta)}p}H_{EFF}}},} & (1)\end{matrix}$

where M_(S) and V are a saturation magnetization and a volume of thefree layer, α is Gilbert's damping constant, p is the spin polarizationof the current, and H_(EFF) is an effective magnetic field acting on thefree layer. The factor g(Θ) depends on the relative angle Θ between themagnetizations of the pinned and free layers.

According to the equation (1) the switching current of the free layercan be effectively controlled by the volume V and/or the magnetizationsaturation M_(S) of the free layer. Besides, the critical currentdepends on the spin polarization p of electrons running through the freelayer and on a mutual orientation of the magnetizations in the free andpinned layers. The dependence of the critical switching current I_(C0)on several parameters provides lot of possibilities for controlling theswitching parameters of the free layer. For instance, to insure anindependent and controllable reversal of the magnetization in the freelayers 112 and 122 affected by the same spin-polarized current I_(S) thelayers should have at least one or several different parameters, such asa layer thickness, the magnetization saturation M_(S) of magneticmaterial, magnetic anisotropy H_(K), etc.

The MR element 31 comprises two free layers 112 and 122. Each of thelayers has two logic states “0” or “1”. Hence the MR element 31 can haveup to four possible logic states that are shown in the FIG. 4. At thecondition of ΔR1=ΔR2, where ΔR1 and ΔR2 is a magnetoresistance of theMTJ elements 11 and 12, respectively, the number of possible logicstates will be reduced up to three since the logical states R1+R2+ΔR1and R1+R2+ΔR2 will be not distinguishable, where R1 and R2 is aresistance of the MTJ elements 11 and 12. To provide the MR element 31with four distinguishable logic states the magnetoresistance of the MTJelements 11 and 12 should be substantially different ΔR1≠ΔR2. Themagnetoresistance ΔR of the MTJ elements significantly depends on athickness and on material properties of the tunnel barrier layers 113and 123. By varying the thickness of the tunnel barrier layers or thematerial of the layers, or both parameters simultaneously the differencein the magnetoresistance of the MTJ elements 11 and 12 can be madesignificant. Besides, the magnetoresistance can be controlled by amaterial selection of the pinned and free layers.

According to the equation (1) the critical current I_(C0) can becontrolled by an angle Θ between the magnetizations in the free andpinned layers. To facilitate a reversal magnetization in the free layer,the layer is made of magnetic material or multilayer that has arelatively low magnetic anisotropy. Hence the orientation of themagnetization in the free layer can be tilted by an external magneticfield applied along a hard axis of the free layer that is resting in thelayer plane. Computed dependence of the normalized switching current onthe angle Θ is given in FIG. 5. The received results suggest that theswitching current can be reduced almost twice at the tilt angle Θ=8degree.

To provide the memory cell 30 with four logic states the tunnel barrierlayers 123 and 113 made of MgO received different thickness, forinstance the layer 113 is 0.9 nm thick and the thickness of the layer123 is 1.2 nm. That will result in of about 100% difference in themagnetoresistance. Similar results can be achieved by using the freelayers 112 and 122 made of different magnetic materials such as CoFeBand CoFe/NiFe at the same thickness of the tunnel barrier layers 113 and123.

The memory cell 30 according to the present invention is using a hybridswitching mechanism. The mechanism assumes a combined effect of the biasmagnetic field and the spin-polarized current simultaneously affectingthe free layer. For instance, the free layer 122 of the MTJ 12 is madeof magnetic material with a lower crystalline anisotropy than that ofthe layer 112. To write a data to the free layer 122 the bias currentI_(B) is applied to the bit line 33. The current I_(B) is running in thevicinity of the MR element 31 but not through the element. The currentI_(B) induces a bias magnetic field H_(B) that is applied along a hardaxis of the layers 122, 32 and 112. Since the pinned layer 32 is made ofa hard magnetic material its magnetization M₃₂ is fixed and does notchange under the effect of the relatively weak magnetic field H_(B).Magnitude of the bias magnetic field H_(B) is inverse proportional to adistance from the bit line 33. Therefore the free layer 122 placedadjacent to the bit line 33 is experienced to the slightly highermagnetic field than the remote free layer 112. However the difference inthe bias field magnitude is small and can be neglected. The bias fieldH_(B) tilts the magnetization M₁₂₂ relatively to its equilibriumposition along an axis 38 that is perpendicular to the layer plane onangle Θ₁₂₂. The tilt angle Θ₁₁₂ is substantially smaller due to thehigher anisotropy (coercivity) of the free layer 112. The transistor 35turns on by applying a voltage to its gate through a word line 34. Thespin-polarized current I_(S) runs through the MR element 31. The currentI_(S) produces a spin momentum transfer from the spin-polarizedelectrons to the free layers 112 and 122. The critical switchingcurrents of the layers 112 and 122 are substantially different due todifferent material properties and/or dimension the layers. The currentI_(S) is insufficient to reverse the magnetization M₁₁₂ in the layer 112but it is strong enough to cause the reversal of the magnetization M₁₂₂in the free layer 122. As a result, the data is written to the layers122 only.

FIG. 6 shows a schematic cross-sectional view of a multi-bit MR element60 according to another embodiment of the present invention. The MRelement includes two pinned layers 111 and 121 separated from each otherby a conductive nonmagnetic spacer layer 62. Thickness of the conductorspacer 62 could be any in a range from 0.5 nm to 100 nm or even higher.At the thickness of the layer 62 of about 0.8 nm made of Ruthenium (Ru)or similar materials the pinned layers 111 and 121 will have a strongantiferromagnetic coupling between each other. At the thickness of thelayer 62 above 5 nm the layers 111 and 121 will be experienced to a weakmagnetostatic coupling. Variation of the layer 62 thickness provides apossibility to control a fringing field produced by the pinned layerthat affect an operation of the free layer.

FIGS. 7A and 7B show a multi-bit MR element 70 with an improvedspin-polarization of the pinned layer 32. The spin-polarization isimportant for reduction of critical current amplitude and for increaseof read back signal of the MR element. The pinned layer 32 includes areference layer 72 with a fixed magnetization oriented substantiallyperpendicular to a layer plane and two spin-polarizing layers 73 and 74positioned between the reference layer 72 and the tunnel barrier layers113 and 123, respectively. The spin-polarized layers are made ofmagnetic material with a high spin-polarization such as CoFe, CoFeB, Feor similar. The layers 73 and 74 have a strong magnetic coupling withthe reference layer 72 to provide them with a substantial perpendicularmagnetization that will be not tilted during write operation under thebias magnetic field H_(B). The spin-polarizing layers 73 and 74 could bemade of a magnetic material with either perpendicular or in-planeanisotropy. To provide a possibility of exchange coupling controlbetween the reference layer 72 and the spin polarizing layers 73 and 74the MR element 70 shown in the FIG. 7B includes thin spacer layers 75and 76, respectively. The layers 75 and 76 are disposed between thereference layer 72 and the spin polarizing layer 73 and 74,respectively. The spacer layers 75 and 76 are made of conductivenonmagnetic materials.

FIG. 8 shows a MR element 80 with a reduced switching current. The freelayer 112 comprises a multilayer structure composed of a soft magneticlayer 82 and a storage layer 81. The storage layer 81 is made ofmagnetic material with perpendicular magnetization and has a substantialmagnetic coupling with the soft magnetic layer 82. The soft magneticlayer 82 is made of a soft magnetic material with, either perpendicularor in-plane anisotropy. The free layer 122 includes a soft magneticlayer 84 and a storage layer 83 having similar properties as the layers81 and 82 of the free layer 112. A perpendicular orientation ofmagnetization in the soft magnetic layer 84 in the equilibrium state isprovided by a strong exchange coupling with the storage layer 83. Thebias magnetic field H_(B) of a relatively low magnitude can cause a tiltof the magnetization in the soft magnetic layer 84 from its equilibriumposition. It will result in a reduction of the spin-polarized currentI_(S) required for magnetization reversal in the reference layer 83 andin the entire free layer 122.

FIG. 9 shows a schematic view of MRAM module 90 comprising an array ofmemory cells 30 shown in FIG. 3, bit line drivers and word line drivers.The MR elements are located at the intersection of parallel conductivebit lines 331, 332 and 33N, with parallel word lines 341, 342 and 34N.Each MR element of the array can be selected individually according to aunique combination of the intersecting bit and word lines in vicinity ofthe element. For instance, to write a data to the MR element 31 locatedat the intersection of the bit line 332 and the word line 342 a pulse ofthe bias current I_(B) of small magnitude is produced in the bit line332 by bit line drivers. The current I_(B) induces a bias magnetic fieldH_(B) along the line 332 and causes a tilt of the magnetization in allMR elements adjacent the bit line 332 creating an issue of ahalf-selected cell. Since the bias field H_(B) has a small magnitude itcannot alone reverse the magnetization in any of the half-selected MRelements disposed along the bit line 332. By applying a voltage pulse toa gate of the selection transistor 35 the MR element 31 located at theintersection of the line 332 and 342 will be selected and a pulse of thespin-polarized current I_(S) will be produced in the MR element 31.Other transistors connected to the word line 342 will be remained closedsince a bias voltage is not applied to other bit lines 331, . . . 33N.Hence a data could be written to the MR element 31 only located at theintersection of the lines 332 and 342. The combined effect of twosuperimposed pulses of the currents I_(B) and I_(S) will cause areversal of the magnetization in the free layer of the MR element 31only. The hybrid switching mechanism that combines a bias magnetic fieldwith a spin-polarized current provides excellent cell selectivity in thearray and low write current.

There is wide latitude for the choice of materials and their thicknesseswithin the embodiments of the present invention.

The pinned layers 32, 111 and 121 have a thickness of about 10-100 nmand more specifically of about 25-50 nm and coercivity measured alongtheir easy axis above 1000 Oe and more specifically of about 3000-5000Oe. The layers 32, 111 and 121 are made of magnetic material withperpendicular anisotropy such as Ni, Fe or Co-based alloys or theirmultilayers such as Co/Pt, Co/Pd, Co/Au, CoFe/Pt, Fe/Pt, Fe/Pd, Ni/Cu orsimilar.

The bit and word lines 33 and 34 are made of Cu, Al, Au, Ag, AlCu,Ta/Au/Ta, Cr/Cu/Cr, poly-Si and similar materials or their basedlaminates.

The seed 36 and cap 37 layers have a thickness of 1-100 nm and morespecifically of about 5-25 nm. The layers are made of Ta, W, Ti, Cr, Ru,NiFe, NiFeCr, PtMn, IrMn or similar conductive materials or their basedlaminates.

The spacer layers 62 is made of conductive nonmagnetic material such asRu, Cu, Re, Ag, Au or similar and their based alloys and laminates. Thelayer 62 has a thickness in a range from 0.5 nm to 100 nm.

The reference layer 72 has a thickness of 10-100 nm and morespecifically of about 20-50 nm; and coercivity above 1000 Oe and morespecifically of about 3000-5000 Oe. The reference layer 72 is made ofmagnetic material with a substantial perpendicular anisotropy such asNi, Fe or Co-based alloys or multilayers such as Co/Pt, Co/Pd, Co/Au,CoFe/Pt, Fe/Pt, Fe/Pd, Ni/Cu or similar.

The spin-polarizing layers 73 and 74 have a thickness of 0.5-5 nm and ahigh spin polarization. They are made of soft magnetic materials with acoercivity of about 1-200 Oe. The spin polarizing layers 73 and 74 aremade of Ni, Fe, Co, their based alloys such as NiFe, CoFe, CoFeB, CoPt,FePt, CoPtCu, FeCoPt and similar or their based laminates such asCoFe/Pt, CoFeB/P and similar. The material of the spin-polarizing layers73 and 74 can have either in-plane or perpendicular anisotropy.

The spacer layers 75 and 76 have a thickness of 0.3-5 nm and morespecifically in a range from 0.5 to 2.5 nm. The spacer layers 75 and 76are made of conductive nonmagnetic materials such as Ru, Cu, Ag, Ag, Reor similar, their based alloys and laminates.

The storage layers 81 and 83 have a thickness of 5-25 nm and morespecifically of about 8-15 nm; and coercivity less than 2000 Oe and morespecifically of about 200-500 Oe. The storage layers 81 and 83 are madeof magnetic materials with a substantial perpendicular anisotropy suchas Fe, Ni or Co-based alloys or multilayers such as Co/Pt, Co/Pd, Co/Au,CoFe/Pt, Fe/Pt, Fe/Pd, Ni/Cu or similar.

The soft magnetic layers 82 and 84 are 0.5-10 nm thick and are made of asoft magnetic material having a substantial spin polarization andcoercivity of about 1-200 Oe such as Ni, Fe, Co-based alloys CoFe,CoFeB, NiFe, Co, Fe, CoPt, FePt, CoPtCu, FeCoPt and similar or theirbased laminates such as CoFe/Pt, CoFeB/P and similar. The materials ofthe soft magnetic layers 82 and 84 can have either in-plane orperpendicular anisotropy.

The free layers 112 and 122 have a thickness of about 1-30 nm and morespecifically of about 5-15 nm and coercivity less than 1000 Oe and morespecifically of about 100-300 Oe. The layer 113 and 123 are made of softmagnetic materials with a perpendicular anisotropy such as Ni, Fe orCo-based alloys or multilayers such as Co/Pt, Co/Pd, Co/Au, CoFe/Pt,Fe/Pt, Fe/Pd, Ni/Cu or similar.

The tunnel barrier layers 113 and 123 have a thickness of about 0.5-25nm and more specifically of about 0.5-1.5 nm. The tunnel barrier layersare made of MgO, Al₂O₃, Ta₂O₅, TiO₂, Mg—MgO and similar materials, theirbased laminates or semiconductors.

It is understood that the above description is intended to beillustrative, and not restrictive. Many other embodiments will beapparent to those of skill in the art upon reviewing the abovedescription. The scope of the invention should be, therefore, determinedwith reference to the appended claims, along with the full scope ofequivalents to which such claims are entitled.

1. A magnetic tunnel junction element comprising: a first free layercomprising a first switching current and a changeable magnetizationoriented substantially perpendicular to a layer plane in its equilibriumstate; a second free layer comprising a second switching current and achangeable magnetization oriented substantially perpendicular to a layerplane in its equilibrium state, the second switching current issubstantially different from the first switching current; a first andsecond tunnel barrier layer, and a pinned layer comprising a fixedmagnetization oriented substantially perpendicular to a layer plane, thepinned layer is disposed between the first and second free layers and isseparated from the free layers by one of the tunnel barrier layers. 2.The magnetic tunnel junction element of claim 1 wherein the free layercomprises at least: a storage layer comprising a first coercivity and amagnetization oriented substantially perpendicular to a layer plane, anda soft magnetic layer comprising a second coercivity and a magnetizationoriented substantially perpendicular to a layer plane in its equilibriumstate, the soft magnetic layer is disposed between the tunnel barrierlayer and the storage layer and is substantially magnetically coupledwith the storage layer; the second coercivity is substantially lowerthan the first coercivity.
 3. The magnetic tunnel junction element ofclaim 2 wherein the soft magnetic layer comprises an in-planeanisotropy.
 4. The magnetic tunnel junction element of claim 1 whereinthe pinned layer comprises at least: a first and second spin polarizinglayer comprising a fixed magnetization oriented substantiallyperpendicular to a layer plane, and a reference layer comprising a fixedmagnetization oriented substantially perpendicular to a layer plane, thereference layer is disposed between the first and second spin polarizinglayers and is substantially magnetically coupled with the first andsecond spin polarizing layers.
 5. The magnetic tunnel junction elementof claim 4 wherein the first and second spin-polarizing layers comprisean in-plane anisotropy.
 6. The magnetic tunnel junction of claim 1wherein the pinned layer comprises: a first pined layer comprising amagnetization oriented substantially perpendicular to a layer plane; asecond pinned layer comprising a magnetization oriented substantiallyperpendicular to a layer plane, and a nonmagnetic spacer layer disposedbetween the first and second pinned layers.
 7. A magnetic memory cellcomprising: a magnetic tunnel junction element including a first andsecond free layer comprising a changeable magnetization orientedsubstantially perpendicular to a layer plane in its equilibrium stateand a switching current, a first and second tunnel barrier layer, and apinned layer comprising a fixed magnetization oriented substantiallyperpendicular to a layer plane, the pinned layer is disposed between thefirst and second free layers and is separated from the free layers byone of the tunnel barrier layers, a selection transistor electricallyconnected to a word line, and a bit line intersecting the word line, themagnetic tunnel junction element is disposed between the bit line andthe selection transistor and is electrically connected to the bit lineand to the selection transistor.
 8. The magnetic memory cell of claim 7wherein the first and the second free layers have substantiallydifferent switching currents.
 9. The magnetic memory cell of claim 7wherein the free layer comprises at least: a storage layer comprising afirst coercivity and a magnetization oriented substantiallyperpendicular to a layer plane, and a soft magnetic layer comprising asecond coercivity and a magnetization oriented substantiallyperpendicular to a layer plane in its equilibrium state, the softmagnetic layer is disposed between the tunnel barrier layer and thestorage layer and is substantially magnetically coupled with the storagelayer; the second coercivity is substantially lower than the firstcoercivity.
 10. The magnetic memory cell of claim 9 wherein the softmagnetic layer comprises an in-plane anisotropy.
 11. The magnetic memorycell of claim 7 wherein the pinned layer comprises at least: a first andsecond spin polarizing layer comprising a fixed magnetization orientedsubstantially perpendicular to a layer plane, and a reference layercomprising a fixed magnetization oriented substantially perpendicular toa layer plane, the reference layer is disposed between the first andsecond spin polarizing layers and is substantially magnetically coupledwith the first and second spin polarizing layers.
 12. The magneticmemory cell of claim 11 wherein the first and second spin polarizinglayers comprise an in-plane anisotropy.
 13. The magnetic memory cell ofclaim 7 wherein the pinned layer comprises: a first pined layercomprising a magnetization oriented substantially perpendicular to alayer plane; a second pinned layer comprising a magnetization orientedsubstantially perpendicular to a layer plane, and a nonmagnetic spacerlayer disposed between the first and second pinned layers.
 14. A methodof writing to a magnetic memory cell comprising: providing a magnetictunnel junction element including a first and second free layercomprising a changeable magnetization oriented perpendicular to a layerplane in its equilibrium state and a switching current, a first andsecond tunnel barrier layer, and a pinned layer comprising a fixedmagnetization oriented substantially perpendicular to a layer plane, thepinned layer is disposed between the first and second free layers and isseparated from the free layers by one of the tunnel barrier layers;driving a bias current pulse through a bit line in a proximity to butnot through the magnetic tunnel junction element for producing a biasmagnetic field along a hard magnetic axis of the pinned, first free andsecond free layers, and driving a switching current pulse through themagnetic tunnel junction element along an easy axis of the pinned, firstfree and second free layers for producing a spin momentum transfer, theswitching current pulse substantially superimposes the bias currentpulse.
 15. The method of claim 14 wherein the free layer comprises atleast: a storage layer comprising a first coercivity and a magnetizationoriented substantially perpendicular to a layer plane, and a softmagnetic layer comprising a second coercivity and a magnetizationoriented substantially perpendicular to a layer plane in its equilibriumstate, the soft magnetic layer is disposed between the tunnel barrierlayer and the storage layer and is substantially magnetically coupledwith the storage layer; the second coercivity is substantially lowerthan the first coercivity.
 16. The method of claim 15 wherein the softmagnetic layer comprises an in-plane anisotropy.
 17. The method of claim15 wherein the magnetization of the soft magnetic layer is tilted fromits equilibrium state by the bias magnetic field.
 18. The method ofclaim 14 wherein the pinned layer comprises at least: a first and secondspin polarizing layer comprising a fixed magnetization orientedsubstantially perpendicular to a layer plane, and a reference layercomprising a fixed magnetization oriented substantially perpendicular toa layer plane, the reference layer is disposed between the first andsecond spin polarizing layers and is substantially magnetically coupledwith the first and second spin polarizing layers.
 19. The method ofclaim 18 wherein the first and second spin polarizing layer comprises anin-plane anisotropy.
 20. The method of claim 14 wherein the pinned layercomprises: a first pined layer comprising a magnetization orientedsubstantially perpendicular to a layer plane; a second pinned layercomprising a magnetization oriented substantially perpendicular to alayer plane, and a nonmagnetic spacer layer disposed between the firstand second pinned layers.